'79 bouncing idle. I'VE TRIED EVERYTHING (kinda)

Can't, you give not enough detail. Not running can mean not even starting to not running because a connecting rod is sticking out of the side of the case.

After a reassembly SOMETHING changed, making the original post no longer relevant.
 
amc49 said:
Can't, you give not enough detail. Not running can mean not even starting to not running because a connecting rod is sticking out of the side of the case.

After a reassembly SOMETHING changed, making the original post no longer relevant.
Click to expand...

Or NOTHING changed. It is still acting the same way. It will run, but not well. The original post is still exactly what it is doing.
 
Compression test and post the results. Throttle held wide open while doing it and preferably more than one test on each cylinder, the reliability of the numbers goes up doing it. You can use a car battery for more reliable result but do NOT have the car running doing it. Both plugs yanked while doing it so engine spins the same.

It makes no sense to chase issues on an engine that is not mechanically sound enough to run right to begin with. Nothing against your work at all but one has to be dispassionate about it to find the problem.
 
A message from my Dad who has been doing the work in Michigan while I am in Tennessee:

"Hi All, I'm new to the "forum world". Forgive me is as I learn the process.

So I'm working on a 79 XS 400. The LH cylinder had a real bad plug cross headed. I removed cover and inserted helicoils in both sides. Lapped the valves and put it all back together. I checked the valves with carb cleaner. They are not leaking. I'm confident things went back together with everything in the correct timing. LH TDC, came shaft and sprocket all aligned. I set the gap and the timing. As part of the rebuild I wrapped pipe tape on the exhaust and attached new mufflers. The mufflers have the same baffle. But the body of the muffler from the baffle to the front end is longer than the mufflers that were on the bike.

Started it up. Used my color tune to adjust the idle. sync the carbs with gauges and check the idle mixture again. Not real happy with the color on the LH. Looked too lean. Idled great. When at about 4000 rpm it would miss. I should also mention that the LH side is much hotter than the RH. Missing at higher rpm so I thought maybe the timing needed to be advanced a little. I moved it forward. Still not luck. RH plug is loading up and LH looks too lean. I opened the carbs up and adjusted the main needle position up leaning out the system. (needle deeper into the main) while in the carbs I checked the jets. I wondered it by some chance they were not the same. Both are 132.5. ( I do have a set of 125) The air jets are both 45. No luck. By the way the other things I tried have been; put the old plugs back in. the new ones were different band just in case the new ones were wrong. And verified the gaps again. No luck. Remove the mufflers to see if the new ones caused some issue. No luck. I'm all out of ideas.

What could I have done to have such an in balance between the two cylinders?

Help - Very frustrated at this point.

Ok. So as I lay awake wondering what was "touched" and wondered about the valve gap. Even though i set them. I grabbed a compression tester. The RH is about 45 psi and the LH side is about 80. The LH plug looks real lean and the RH side is too rich. There is nothing i could do to affect that condition. Could there be some issue with the seal of the head and cover. I put in a new gasket at the head and liquid gasket for the cover.
Anybody have any direction?

Thanks"
 
amc49 said:
Compression test and post the results. Throttle held wide open while doing it and preferably more than one test on each cylinder, the reliability of the numbers goes up doing it. You can use a car battery for more reliable result but do NOT have the car running doing it. Both plugs yanked while doing it so engine spins the same.

It makes no sense to chase issues on an engine that is not mechanically sound enough to run right to begin with. Nothing against your work at all but one has to be dispassionate about it to find the problem.
Click to expand...
Do you know what my compression numbers SHOULD be?
 
Yes but not going to state until you get them. Too often what people think they what should be creeps into what is supposed to be reality in the reporting of them. Why the reliability of them is so suspect among other things.

I WILL say this, if not well over 100 psi you have problems with the test or the engine. 100 being a solid point at which most engines car or bike will not burn hot enough to keep the plugs from fouling. More compression increases burn heat. I worked that number out many years ago and have never seen it disproven yet if it is a real number. When compression gets low the idle will get quite erratic too, the only thing keeping it going to not die is then crank inertia. Sudden dies should begin to show up too, like clutching for a slowdown decel and then bike dying suddenly with the rpm drop.
 
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Sounds like you’ve done most of the right stuff. What’s the mileage? Have you had the head off and looked at the barrels? Did you get a compression measurement? Have you changed the gas? Got any photos?
These things take time and patience but when you get them sorted u soon forget what the fuss was about
 

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